Build your own flat panel solar thermal collector

Step 8 Why does this collector design work so well?

Most home brew and commercial solar collector designs I have seen use metal (usually copper) tubing to carry the water through the panel. Metal fins are attached to the copper tubing. The fins are painted black. The fins heat up and conduct the heat to the tubing. Metal is a good conductor, but the heat has to travel a long way through a thin cross-section to reach the tubing. In my design, I used plastic which is a poor conductor, but the heat only has to travel about 0.3mm through a very large cross-section from the front surface of the panel to the water. I'll illustrate why this is better.

There is a property of any thermal system called thermal conductance that indicates how much heat (power) can be transfered from point 'a' to point 'b' for a given temperature differential. The formula is:

Thermal Conductance = K * A / L
where:
K = thermal conductivity (a physical property of the material)
A = cross-sectional area through which heat must travel
L = distance heat must travel (the distance from 'a' to 'b').

Lets calculate the thermal conductance of a typical flat panel collector.

Assume the panel is 2'x8' with 4 copper tubes running lengthwise and fins sticking out 3" on either side of every tube (6" per tube x 4 tubes fills our 2' width). Suppose the fins are 1mm thick and also made of copper. When the fins heat up, the cross sectional area through which this heat must be conducted to reach the tubes is 1mm * 8 ft * 8 fins = 2438 mm². The average distance the heat must be conducted is 1/2 the fin width or 1.5" = 38 mm. The conductivity of copper is about 0.4 W/mm/degreeC.

Therefore the thermal conductance from the collector surface to the water is 0.4 W/mm/degreeC * 2438 mm² / 38 mm = 25W/degreeC. In other words, a 1 degreeC temperature difference between the water and the fin will result in 25W of heat transfer. But the panel is receiving something on the order of 1400 W of incoming power from sunlight. To transfer all that power to the water by conduction alone the fins would have to heat up to 56 degrees C higher than the water temperature.

Now repeat the calculation for the corrugated plastic panel.

The cross sectional area through which heat must be conducted is the receiving area of the panel itself (2' * 8' = 1486448 mm²). The distance the heat must travel to reach the water is just the thickness of the plastic wall or about 0.3mm. The conductivity of plastic is about 0.0001 W/mm/degreeC. Note that it is over 1000 times lower than copper which makes sense since plastic is general thought of as an insulator, not a conductor.

Therefore the thermal conductance of the system is 0.0001 W/mm/degreeC * 1486448 mm² / 0.3 mm = 495 W/degreeC. In other words, a 1 degreeC temperature difference between the water and the collector surface will result in 495 W of heat transfer into the water. To transfer 1400W, the panel surface only needs to heat up about 3 degreesC hotter than the water.

Of course in practice, not all of that 1400W goes into the water. The conductance from the collector surface to the water is in parallel with another conductance from the collector surface to the outside air. The relative values of those two conductances determines how much heat goes where (Aside: this is analogous to current in an electrical circuit with two resistors in parallel.)

Conclusion
You can see that in spite of the much lower thermal conductivity of plastic, using a corrugated plastic sheet as a collector achieves 20 times higher conductance between the collector surface and the water when compared to a typical tube-and-fin design.

If a corrugated collector could be made from copper, the results would be even better, but not very much better, for reasons I won't go into because I can already feel everyone's eyes glazing over.

Thanks for reading. For information on this and other projects of mine see my website IWillTry.org.

Comments

[wolfkeeper]

For permanent installs, wouldn't a steel central heating radiator used as a heat exchanger, with double glazing on top, work nearly as well or better and avoid the problem of potentially melting?

[phredder]

You did a nice job on your thermal collector. I built a similar design several years ago to heat my kids pool that was 12 feet in diameter and 3.5 feet high. Mid summer the pool temperature would get up to around the 90 degree mark, without the collector the temp would be below 70. I used the full 8 x 4 black sheet if coraplast and used 1.5 inch a.b.s. for the ends. The a.b.s. ends were 4.5 feet in length. I didn't make a continues cut along the length of the a.b.s., rather I started my cut about two inches from the end, then I'd cut an 11 inch cut, leave another uncut portion of about 3/4 inch, make another 11 inch cut, leave another portion of material uncut, and so-on down the length of pipe until I got to 48 inches, I then left the last 6 inches uncut. Before I slid my coraplast in the a.b.s. I had to notch the coraplast to fit the uncut portions of the pipe. My reason for this type of slotting was to add strength to the pipe to resist the internal pressure of the water that was being pushed by a 1.5 h.p. pool pump. The hose I was using was 1.5 inch black sump pump hose. Not the most attractable pool hose but functional. My biggest concern was what adhesive to use to join the pipe and coraplast. At that time the only info I could find on coraplast was from the manufactures data sheets and modelers/hobbyists who use coraplast to build their projects. I did used silicone but next time plan to use urethane, the stuff the automotive industry uses to adhere windshields to vehicles. the was fastened to a 8x4 sheet of plywood and then the whole assemble screwed to my shed roof that face south. The collector remained on the roof for 3 years and withstood everything except the cats claws.

[paladin42]

neat concept. would like to know how to apply this to home use. can you use your existing hot water heater as a storage tank and can you hook it up directly to your water line. if yes to the former how do you keep the stored water hot until use?

[iwilltry]

You could use your existing hot water heater as a storage tank. You could not put this system directly in line though. City water pressure would burst the joints. You would required a heat exchanger in the tank (do a web search on Solar Wand) and a pump to circulate the water when it is sunny. The tank keeps the water hot till use (it's insulated). During non-sunny periods you can run the tank normally. A well built system would switch automatically between normal and solar operation as needed.

[votecoffee]

You could use this with an existing system, but you are right in that you would need a check valve on the output and a pressure regulator on the input side that would greatly reduce the pressure.  If you can not find a pressure regulator that will bring the pressure down sufficiently, you could use a storage tank and float valve to fill the tank to a set height.  Either way, you will need a pump to get the water pressure up higher than house pressure for the check valve to open and let water in.

[UK_Westy]

If the storage container was below the height of the solar collector, I assume a pump could be used to pump it back up to the top and let gravity take it's cause ? Do you think a drill pump fitted to a rechargable hand drill running off a solar panel work ?

[iwilltry]

Yes. It is usually much more convenient for the solar collector to be mounted higher than the storage container. Therefore a pump must be used to circulate the water. A drill pump may work, but I don't know if they are designed for continuous duty, and I don't think they are very efficient (ie you may require a bigger solar panel than you think). If you have access to AC power, I would recommend using that instead of adding photovoltaics. You might try an AC aquarium pump or a sump pump. But then you either have to turn it on and off manually, or design a thermostat to turn it on when the collector is hotter than the storage container.

[Bullmoose]

Awesome Article! Thanks! What kind of plastic is the panel? I wonder if it's one of those types that leeches poisonous chemicals into the water. Not a problem though if one were to use a heat exchanger rather than attempting to directly heat the potable water. Also, how long do you think it will take for the plastic to breakdown, turn brittle, start leaking, etc...? Keep it up.. This is great stuff!

[iwilltry]

Much of that information can be found on my own website IWillTry.org. Scroll down to the comments where there is some discussion of materials and lifetime. Thanks. -Rob

[kas83]

any ideas how to convert this cheaply to an electricity?

[WTHAI]

You could build a few Stirling engines. there's an instructable on here somewhere about how to build one.

[bladetiff]

This seemed like a really understandable project but how do I get use of the water and would it be possible to use this to get hot water into inground pool to extend the pool season a little on each end

[iwilltry]

This type of panel should work well for pool heating. In the case of an in ground pool, the panels must be located higher than the pool, so you must use a pump to circulate water through them. You won't be able to rely on thermo-syphoning. Depending on the size of the pool you may need a lot of panels to see a significant result. Because water won't flow with the pump turned off, it will be easier for the panels to overheat and possibly melt. Therefore, I would recommend not to install the panels in an insulated frame with transparent cover. Just lay them directly on a roof or some surface exposed to the sun.

[stone1716]

I am new to the site. I make signs, like you talked about. So this is very interesting to me. You can also purchase 10mm corplast in black (I pay $24 US, for 4'x8') instead of 3mm white. Some of the corplast signs we have used, have withstood 24/7 weather conditions, for 4 + years now. They are the 3mm type, the 10mm should last much longer. They are not only thicker in width but the pvc walls are thicker also. The corrugations are larger too (do not know how that would effect the heat ratio? and if it would out-way the life of replacement?). I will be testing the project in the near future, I have to admit complete ignorance, as to how to determine any effenciency?

[iwilltry]

A 10mm panel should work just as well as a 3mm panel. The main consideration is heat loss which is roughly proportional to panel surface area. Changing the thickness from 3mm to 10mm doesn't significantly increase the surface area so it should have little effect. It may even perform better because the wider channels may result in a higher thermo-syphon flow rate. However, it would be over 3 times heavier when filled with water so it would need to be well supported.

[das300]

This can be improved but its a great idea. I have seen one in youtube that uses a felt material and black building plastic to cover the felt and then plastic corrugated sheet over it making it very applicable for a roof. you can adapt that design into this one to make improvements as well as some safety adds in case it over heats or leaks ( gutter system under plastic using building plastic and a lower gutter ) . Well done though ..

[Leroy]

I've been reading instructables for several years now. Yours is the best so far. Thanks for posting it. Everybody who lives in California should make one of these right now.

[pangavamanos]

Wow! I sure enjoyed your project. Has me thinking of ideas for my on test. BTW Wasn't glazing over. Didn't understand it all but that is ok. Learned a ton. Thanks JL